/Engine/Source/Runtime/Core/Public/Containers/BitArray.h
Finds the first occurrence of a zero bit in the array and sets it to one, returning the bit index. If there are no zero bits in the array, INDEX_NONE is returned.
The index of the first occurrence of a zero bit that was successfully set to one, or INDEX_NONE if not found.
int32 FindAndSetFirstZeroBit ( int32 StartIndex )